Δ-Σ modulator and Δ-Σ A/D converter

ABSTRACT

A Δ-Σ converter comprises a Δ-Σ modulator and a digital filter. The Δ-Σ modulator has a comparator having two resonant tunnel diodes connected to each other in series between two terminals and a field effect transistor connected in parallel to one of the resonant tunnel diodes, a conversion input transistor for converting an input voltage into electric current, a capacitor provided so that charge amount thereof is decreased by flow of electric current through the conversion input transistor and an electric potential proportional to the charge amount is used for the input electric potential to the comparator, and a feedback transistor, to which an output of the comparator is inputted, operating in such a way as to increase the charge amount of the capacitance element when the output of the comparator becomes a high level.

BACKGROUND OF THE INVENTION

The present invention relates to a DELTA-SIGMA (hereinafter referred toas Δ-Σ) modulator and a Δ-Σ A/D converter having the Δ-Σ modulator. Thepresent invention is applicable to ultra-high communication apparatusessuch as software radio and high-frequency measuring apparatuses.

An A/D (analog-digital) converter is a circuit having a function forconverting an analog signal into a digital signal and indispensable asan interface between an actual world indicated by analog information anda digital signal processing circuit.

As the conventional A/D converter, there are mainly two types of A/Dconverter such as the flash type and the Δ-Σ type. In the flash-type A/Dconverter, an input voltage is divided by means of many resistances, anddivided voltages are applied in parallel to comparators to convert aninput analog value to a digital value. Therefore, the flash-type A/Dconverter is capable of operating at a very high speed but incapable ofcoping with variations of characteristics of elements and has difficultyin obtaining high precision (number of bits).

On the other hand, in the Δ-Σ A/D converter, an input signal isconverted into a digital signal of low bit precision at a very highsampling rate. Then a signal having a required frequency is taken outfrom the digital signal and then high-precision digital data is obtainedby converting precision in time base direction into precision in bit.The circuit is capable of coping with variation in a device parameterand making high-precision conversion. The operation principle of the Δ-ΣA/D converter is described below based on a document (“A Simple Approachto Digital Signal Processing” written by C. Marven and G. Ewers;translated by Hiroshi Yamaguchi, published by Maruzen). FIG. 1 shows ablock diagram of the Δ-Σ A/D converter.

The Δ-Σ A/D converter comprises a Δ-Σ modulator (Δ-Σ quantizer) and adigital filter. The Δ-Σ modulator samples an input signal at a very highfrequency and coverts the sampled input signal into a pulse densitysignal. The digital filter decreases a high-frequency component of thesignal and converts precision in time base direction into precision inbit to thereby take out a digital signal having high precision.

FIG. 2 shows the simplest construction of the Δ-Σ modulator which is themain part of the Δ-Σ A/D converter. This circuit comprises an integratorand a comparator. This circuit integrates the deviation between an inputsignal and an output signal and controls an output in such a way as tominimize the deviation. The output signal becomes a pulse signal. As aresult, the pulse density is so controlled as to be coincident with aninput value, and a pulse density signal proportional to the input valueis obtained. At this time, by setting the frequency of a clock signalfor sampling much higher than a Nyquist frequency, the A/D converter iscapable of obtaining high precision.

As described above, because this circuit has high precision byincreasing the sampling rate, the circuit of the Δ-Σ A/D converter hasadvantages that it is capable of coping with variations of a circuitparameter than the flash-type A/D converter and does not require ahigh-precision analog element in obtaining high precision.

In the Δ-Σ A/D converter having the above-described characteristics, avery high sampling frequency is necessary for a frequency of a signal tobe obtained. Thus the conventional art constructed of a transistor, anoperational amplifier, and the like has a problem that it can be usedonly for an audio signal having a comparatively low frequency or thelike.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel Δ-Σmodulator which has overcome the problem of the conventional art and aΔ-Σ A/D converter using the Δ-Σ modulator.

The Δ-Σ modulator according to the present invention has a comparatorhaving a plurality of negative resistance elements which show an N-typedifferential negative resistance and are connected to each other inseries between two terminals and a comparison input transistor connectedin parallel to at least one of the negative resistance elements, a clockpulse voltage being applied between the two terminals; a conversioninput transistor for converting an input voltage into electric current;a capacitance element connected to an output current terminal of theconversion input transistor and to an input terminal of the comparisoninput transistor of the comparator; and a feedback transistor to whichan output of the comparator is inputted and through which electriccurrent flows into the capacitance element in a direction opposite to adirection of electric current flowing through the conversion inputtransistor. The Δ-Σ modulator is capable of achieving a high-speedoperation, unlike the conventional one.

The above-described negative resistance element can be a resonant tunnelelement such as a resonant tunnel diode or other elements showing adifferential negative resistance such as an Esaki diode. The transistorcan be a field effect transistor, a bipolar transistor or the like.

Further, the present invention is characterized in that the Δ-Σ A/Dconverter comprises the above-described Δ-Σ modulator and a digitalfilter. According to this invention, it is possible to realize ahigh-precision A/D converter operating at an ultra-high frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the construction of a Δ-Σ A/D converter.

FIG. 2 shows the construction of a conventional Δ-Σ modulator.

FIG. 3 shows the construction of a Δ-Σ modulator according to a firstembodiment of the present invention.

FIG. 4 shows input-output characteristics of the Δ-Σ modulator of FIG.3.

FIG. 5 shows the construction of a Δ-Σ modulator according to a secondembodiment of the present invention.

FIG. 6 shows the construction of another example of a comparator shownin FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

FIG. 3 shows the construction of a Δ-Σ modulator according to a firstembodiment of the present invention. The Δ-Σ modulator has a comparatorhaving two resonant tunnel diodes RTD1, RTD2 connected in series witheach other between two terminals (clock terminal and ground terminal)and a field effect transistor Tr3 connected in parallel with theresonant tunnel diode RTD1, an input field effect transistor Tr1, acapacitor (capacitance element) C serving as an integrator, and afeedback field effect transistor Tr2.

A clock pulse voltage is applied between the clock terminal and theground terminal. An input voltage Vin is inputted to the inputtransistor Tr1.

A resonant tunnel element indicated as the resonant tunnel diodes RTD1,RTD2 is a negative resistance element showing an N-type (voltage controltype) differential negative resistance in a current-voltagecharacteristic.

The comparator having the resonant tunnel diodes RTD1, RTD2 and thefield effect transistor Tr3 is known as a monostable-bistable transferlogical element (MOBILE) and has the characteristic of a high-speed, anedge trigger (output is determined by input value at rise time ofclock), and a latch operation (value is held while clock has highlevel). In the embodiment, the input transistor Tr3 is connected inparallel with the resonant tunnel element RTD1 disposed at the lowerside (electric potential-fixed terminal side). Thus, the comparatorperforms an operation of inverter type. That is, when an input electricpotential to the transistor Tr3 is higher than a threshold, thecomparator outputs a low-level pulse signal, whereas when the inputelectric potential thereto is lower than the threshold, the comparatoroutputs a high-level pulse signal.

In this circuit, the input transistor Tr1 is biased in a saturatedregion. Accordingly, irrespective of an electric potential Vc at a pointA, electric current I1 proportional to the input voltage Vin flowsthrough the input transistor Tr1. A drain of the input transistor Tr1 isconnected to the capacitor C. Thus when the electric current I1 flowsthrough the input transistor Tr1, a charge amount stored in thecapacitor C decreases. Thereby the charge amount of the capacitor Cdecreases at a speed proportional to the input voltage Vin, and theelectric potential (electric potential Vc at point A) proportional tothe charge amount drops at a speed proportional to the input voltageVin. When the electric potential Vc at the point A is higher than thethreshold of the comparator, an output Vout of the comparator becomes alow level. When. the electric potential Vc at the point A is lower thanthe threshold thereof, the output Vout thereof becomes a high level.

When the output Vout of the comparator becomes a high level, thefeedback transistor Tr2 is turned on. Thereby electric current flowsinto the capacitor C. The direction of this electric current is oppositeto the direction of electric current flowing through the inputtransistor Tr1. Thereby the charge amount stored in the capacitor Cincreases, the electric potential Vc at the point. A rises, and theoutput Vout of the comparator becomes a low level. This indicates that anegative feedback is generated, and this circuit operates as the Δ-Σmodulator.

For example, when the input voltage Vin is high, the drop speed of theelectric potential Vc at the point A is fast. Thus at a sampling time,there is an increase in the number of times when the electric potentialVc at the point. A becomes lower than the threshold, and the comparatoroutputs many pulses. On the other hand, when the input voltage Vin islow, the drop speed of the electric potential Vc at the point A is slow.Thus at the sampling time, there is an increase in the number of timeswhen the electric potential Vc at the point A becomes higher than thethreshold and the pulse density becomes low.

FIG. 4 shows the result of simulation of the input-output characteristicof this circuit. As shown in FIG. 4A, when the input voltage Vin ischanged linearly with respect to time, the pulse density of the outputVout of the comparator changes in proportion to the input voltage Vin,as shown in FIG. 4B. Accordingly, by connecting a digital filter to theΔ-Σ modulator shown in FIG. 3, a high-speed and high-precision A/Dconverter can be obtained. The digital filter may be a conventionaldigital filter. But the digital filter operating at a higher speed thanthe conventional digital filter is preferable. For example, the digitalfilter constructed by MOBILE using the resonant tunnel element canoperate at a high speed similarly to the Δ-Σ modulator.

(Second Embodiment)

FIG. 5 shows the construction of a Δ-Σ modulator according to a secondembodiment of the present invention. The Δ-Σ modulator of the secondembodiment is different from the Δ-Σ modulator of the first embodimentin that the input transistor Tr3 of the comparator is connected inparallel to the resonant tunnel element RTD2 disposed at the upper side.In this case, when an input electric potential is higher than athreshold, the comparator outputs a high-level pulse signal, whereaswhen the input electric potential is lower than the threshold, thecomparator outputs a low-level pulse signal. Therefore the connectionrelationship between the input transistor Tr1 and that of the feedbackfield effect transistor Tr2 are opposite to those of the firstembodiment. Accordingly in the circuit, an operation in which the highlevel of the voltage and the low level thereof in the second embodimentare reverse to those in the first embodiment is performed. As the outputof the circuit, a pulse density signal similar to that of the firstembodiment is obtained.

Since the resonant tunnel element is used for the Δ-Σ modulator of thefirst and second embodiments, the Δ-Σ modulator has the following manyadvantages: (1) the Δ-Σ modulator can be constructed with a simplecircuit and thus the circuit area thereof can be reduced; (2) the Δ-Σmodulator consumers a low electric power; (3) the Δ-Σ modulator iscapable of operating at a high speed; and (4) the integration can beaccomplished easily.

The Δ-Σ A/D converter having the Δ-Σ modulator can be used as thehigh-precision A/D converter operating at an ultra-high frequency (forexample, two-digit GHz). For example, the Δ-Σ A/D converter can beutilized for high-precision A/D conversion of sounds and images and A/Dconversion. for a communication broad band modem, an RF band and a baseband of a software radio.

(Other Embodiments)

In the above-described embodiment, the resonant tunnel element is usedas the negative resistance element. But other elements such as an Esakidiode showing a negative resistance may be used. The transistor is notlimited to the field effect transistor but a bipolar transistor may beused as the transistor.

Further, as described in a document “Proceedings of 27th InternationalSymposium on Multiple-Valued Logic, 1997, pp. 35-40”, the comparatorobtaining a multi-valued output can be composed by increasing the numberof negative resistance elements connected to each other in series. Forexample, the comparator of the first embodiment may be constructed asshown in FIG. 6. That is, resonant tunnel elements RTD11—RTD13 andRTD21—RTD23 are connected to each other in series respectively, and thetransistor Tr3 is connected in parallel to the resonant tunnel elementsRTD11—RTD13. In case of the comparator of the second embodiment, thetransistor Tr3 is connected in parallel to the resonant tunnel elementsRTD21—RTD23.

What is claimed is:
 1. A Δ-Σ modulator comprising: a comparator having aplurality of negative resistance elements which show an N-typedifferential negative resistance and are connected to each other inseries between two terminals and a comparison input transistor connectedin parallel to at least one of said negative resistance elements, aclock pulse voltage being applied between said two terminals; aconversion input transistor for converting an input voltage intoelectric current; a capacitance element connected to an output currentterminal of said conversion input transistor and to an input terminal ofsaid comparison input transistor of said comparator; and a feedbacktransistor to which an output of said comparator is inputted and throughwhich electric current flows into said capacitance element in adirection opposite to a direction of electric current flowing throughsaid conversion input transistor.
 2. A Δ-Σ modulator according to claim1, wherein the number of said negative resistance elements connected inseries to each other between said two terminals is two, and saidcomparison input transistor is connected in parallel to one of twonegative resistance elements.
 3. A Δ-Σ modulator according to claim 1,wherein said negative resistance element is a resonant tunnel element.4. A Δ-Σ modulator according to claim 1, wherein each of said comparisoninput transistor, said conversion input transistor, and said feedbacktransistor is a field effect transistor.
 5. A Δ-Σ modulator comprising:a comparator having two negative resistance elements which show anN-type differential negative resistance and are connected to each otherin series between two terminals and a comparison input transistorconnected in parallel to one of said two negative resistance elements, aclock pulse voltage being applied between said two terminals, and saidcomparator outputting a low-level pulse signal when an input electricpotential is higher than a threshold and outputting a high-level pulsesignal when said input. electric potential is lower than said threshold;a conversion-input transistor, for converting an input voltage intoelectric current, having an output current terminal connected to aninput terminal of said comparison input transistor; a capacitanceelement provided so that charge amount thereof is decreased by flow ofelectric current through said conversion input transistor and anelectric potential proportional to said charge amount is used for saidinput electric potential to said comparator; and a feedback transistor,to which an output of the comparator is inputted, operating in such away as to increase the charge amount of the capacitance element when theoutput of the comparator becomes a high level.
 6. A Δ-Σ modulatoraccording to claim 5, wherein said negative resistance element is aresonant tunnel element.
 7. A Δ-Σ modulator according to claim 5,wherein each of said comparison input transistor, said conversion inputtransistor, and said feedback transistor is a field effect transistor.8. Δ-Σ modulator comprising: a comparator having two negative resistanceelements which show an N-type differential negative resistance and areconnected to each other in series between two terminals and a comparisoninput transistor connected in parallel to one of said two negativeresistance elements, a clock pulse voltage being applied between saidtwo terminals, and said comparator outputting a low-level pulse signalwhen an input electric potential is higher than a threshold andoutputting a high-level pulse signal when said input electric potentialis lower than said threshold; a conversion input transistor, forconverting an input voltage into electric current, having an outputcurrent terminal connected to an input terminal of said comparison inputtransistor; a capacitance element provided so that charge amount thereofis decreased by flow of electric current through said conversion inputtransistor and an electric potential proportional to said charge amountis used for said input electric potential to said comparator; and afeedback transistor, to which an output of the comparator is inputted,operating in such a way as to increase the charge amount of thecapacitance element when the output of the comparator becomes a highlevel.
 9. A Δ-Σ modulator according to claim 8, wherein said negativeresistance element is a resonant tunnel element.
 10. A Δ-Σ modulatoraccording to claim 8, wherein each of said comparison input transistor,said conversion input transistor, and said feedback transistor is afield effect transistor.
 11. A Δ-Σ converter comprising: a comparatorhaving a plurality of negative resistance elements which show an N-typedifferential negative resistance and are connected to each other inseries between two terminals and a comparison input transistor connectedin parallel to at least one of said negative resistance elements, aclock pulse voltage being applied between said two terminals; aconversion input transistor for converting an input voltage intoelectric current; a capacitance element connected to an output currentterminal of said conversion input transistor and to an input terminal ofsaid comparison input transistor of said comparator; a feedbacktransistor to which an output of said comparator is inputted and throughwhich electric current flows into said capacitance element in adirection opposite to a direction of electric current flowing throughsaid conversion input transistor; and a digital filter to which anoutput of said comparator is inputted.
 12. A Δ-Σ converter according toclaim 11, wherein the number of said negative resistance elementsconnected in series to each other between said two terminals is two, andsaid comparison input transistor is connected in parallel to one of twonegative resistance elements.
 13. A Δ-Σ converter according to claim 11,wherein said negative resistance element is a resonant tunnel element.14. A Δ-Σ converter according to claim 11, wherein each of saidcomparison input transistor, said conversion input transistor, and saidfeedback transistor is a field effect transistor.
 15. A Δ-Σ convertercomprising: a comparator-having two negative resistance elements whichshow an N-type differential negative resistance and are connected toeach other in series between two terminals and a comparison inputtransistor connected in parallel to one of said two negative resistanceelements, a clock pulse voltage being applied between said twoterminals, and said comparator outputting a low-level pulse signal whenan input electric potential is higher than a threshold and outputting ahigh-level pulse signal when said input electric potential is lower thansaid threshold; a conversion input transistor, for converting an inputvoltage into electric current, having an output current terminalconnected to an input terminal of said comparison input transistor; acapacitance element provided so that charge amount thereof is decreasedby flow of electric current through said conversion input transistor andan electric potential proportional to said charge amount is used forsaid input electric potential to said comparator; and a feedbacktransistor, to which an output of the comparator is inputted, operatingin such a way as to increase the charge amount of the capacitanceelement when the output of the comparator becomes a high level; and adigital filter to which an output of said comparator is inputted.
 16. AΔ-Σ converter according to claim 15, wherein said negative resistanceelement is a resonant tunnel element.
 17. A Δ-Σ converter according toclaim 15, wherein each of said comparison input transistor, saidconversion input transistor, and said feedback transistor is a fieldeffect transistor.
 18. A Δ-Σ converter comprising: a comparator havingtwo negative resistance elements which show an N-type differentialnegative resistance and are connected to each other in series betweentwo terminals and a comparison input transistor connected in parallel toone of said two negative resistance elements, a clock pulse voltagebeing applied between said two terminals, and said comparator outputtinga high-level pulse signal when an input electric potential is higherthan a threshold and outputting a low-level pulse signal when said inputelectric potential is lower than said threshold; a conversion inputtransistor, for converting an input voltage into electric current,having an output current terminal connected to an input terminal of saidcomparison input transistor; a capacitance element provided so thatcharge amount thereof is decreased by flow of electric current throughsaid conversion input transistor and an electric potential proportionalto said charge amount is used for said input electric potential to saidcomparator; and a feedback transistor, to which an output of thecomparator is inputted, operating in such a way as to increase thecharge amount of the capacitance element when the output of thecomparator becomes a high level.
 19. A Δ-Σ converter according to claim18, wherein said negative resistance element is a resonant tunnelelement.
 20. A Δ-Σ converter according to claim 18, wherein each of saidcomparison input transistor, said conversion input transistor, and saidfeedback transistor is a field effect transistor.